My personal opinion is that, if POIS was due mainly to overmasturbation, it would not be a rare syndrome, unknown to most doctors, but a very common problem. So I think there must be something that predispose to have POIS, and overmasturbation may be one of the possible triggers, but not the only one.
For me, I never had overmasturbation problem, so not the cause of my POIS.